I may be dumb but what is 20XX/C-Stick

Using 20XX mods and/or C-Stick for single player modes.

Got it

So you use a mod that enables the C-Stick in single player play?

Yes, Super Smash Bros. Melee modifications like making the C-Stick perform attacks in single player mode are popular with a bunch of players. There are numerous pieces of software out there that let players have this functionality. This modified feature, along with other modifications, gave this game enough changes and new possibilities that we wanted to create a separated game to have competition for such concepts. People who like these changes and/or don't like the original game's single player implementations can now put their speedruns in a dedicated area that doesn't intrude upon the normal game's style or history. If you want to discover more about these different alterations to Super Smash Bros. Melee, you can search the world wide web for details.

Alright sounds cool, I may try and find some and run them

Do you have a link to a mod? I couldn’t find anything specific. Specifically something that could patch to a .iso file?

I don't have any resources like that or information about modifications or altered versions of Super Smash Bros. Melee. I don't have any experience with this sort of field, but I think slippy318 knows about such things and could help you. He is the leading moderator for this Category Extension and has an idea of what this game will be like.

Is it ok to use faster melee slippi? (The one everyone is using for the unranked netplay nowadays.) It has singleplayer but I'm not sure if the cpus are normal.

@GoodleShoesGoodleShoes From what i understand. Faster Melee slippi is mostly an emulator and you can use the original Melee iso with that. However since this is category extensions. You could also use a 20XX iso with altered cpu behaviour as long as it isn't set to settings that would reduce RNG. Like manipulated cpu DI behaviour and such. But yes you can use any Dolphin emulator since it doesn't change the iso at all. Even if load times would be faster on some emulators. It wouldn't change anything since most of the categories are timed with In Game Time except multiple game category runs like All trophies and All characters

Thanks for the answer!
Edit: I just tried it out and it might not be the best for running, I experienced some things like opponents instantly dying, cpus all spawning in the same place and on normal mode crazy hand appeared.

@GoodleShoesGoodleShoes Hmm. As for Crazy Hand. He usually appears on Normal difficulty or higher if you don't game over at least once first (Most speedrunners game over on purpose on the Metal stage to avoid fighting Crazy Hand). Opponents instantly dying may be the cpu's messing up. If you mean they randomly jump off the stage. DK is a great example of that. On Adventure mode Very Hard you can step a tiny bit forward and crouch. Big DK will walk up to you. Neutral air and just die in the water. CPU's all spawning in the same place however might be an emulator issue. I can't think of many places that could happen in the vanilla game. Tho i know custom stages often makes that happen. I do know that for Slippi Rollback to work. It has to replace some kind of mechanic about Melee to fit the whole Rollback thing in. And if they would mess with the CPU then that makes a lot of sense since you normally don't play CPU's with that version of Dolphin. But thank you so much for letting me know.
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I tried slippi rollback and regular dolphin with cpu's. Went on Final Destination and had player 1 and 2 be a team and player 3 and 4 be a team. I did notice some differences in the placements of how they spawn. Seems like Slippy always spawns the teams on each side so P1 and P2 is on the left and P3 and P4 is on the right side of FD. On regular Dolphin. The cpu's were scattered randomly. So the slippy emulator/faster melee emulator does change cpu spawn locations